1) the kernel (i don't know if client or server) MUST put some error message
in the syslog like "nfs client: can only transfer 16 member groups. dropping
some groups. expect random failures. Add --manage-gids to rpc.mountd" or so.
Just a hint that there is something going wrong and what to google for!
instead you are left alone without any kind of information at all. that's
really bad!
2) --manage-gids should be default option
3) nfs in debian-etch should also support --manage-gids, not just in debian
lenny.